547 research outputs found
Scenarios for ultrafast gamma-ray variability in AGN
We analyze three scenarios to address the challenge of ultrafast gamma-ray
variability reported from active galactic nuclei. We focus on the energy
requirements imposed by these scenarios: (i) external cloud in the jet, (ii)
relativistic blob propagating through the jet material, and (iii) production of
high-energy gamma rays in the magnetosphere gaps. We show that while the first
two scenarios are not constrained by the flare luminosity, there is a robust
upper limit on the luminosity of flares generated in the black hole
magnetosphere. This limit depends weakly on the mass of the central black hole
and is determined by the accretion disk magnetization, viewing angle, and the
pair multiplicity. For the most favorable values of these parameters, the
luminosity for 5-minute flares is limited by ,
which excludes a black hole magnetosphere origin of the flare detected from
IC310. In the scopes of scenarios (i) and (ii), the jet power, which is
required to explain the IC310 flare, exceeds the jet power estimated based on
the radio data. To resolve this discrepancy in the framework of the scenario
(ii), it is sufficient to assume that the relativistic blobs are not
distributed isotropically in the jet reference frame. A realization of scenario
(i) demands that the jet power during the flare exceeds by a factor the
power of the radio jet relevant to a timescale of years.Comment: 15 pages, accepted by Ap
Infrared afterglow of GRB041219 as a result of reradiation on dust in a circumstellar cloud
Observations of gamma ray bursts (GRB) afterglows in different spectral bands
provide a most valuable information about their nature, as well as about
properties of surrounding medium. Powerful infrared afterglow was observed from
the strong GRB041219. Here we explain the observed IR afterglow in the model of
a dust reradiation of the main GRB signal in the envelope surrounding the GRB
source. In this model we do not expect appearance of the prompt optical
emission which should be absorbed in the dust envelope. We estimate the
collimation angle of the gamma ray emission, and obtain restrictions on the
redshift (distance to GRB source), by fitting the model parameters to the
observational data.Comment: 6 pages, 2 figures, Submited to Astrofizik
Large-scale flow dynamics and radiation in pulsar gamma-ray binaries
Several gamma-ray binaries show extended X-ray emission that may be
associated to interactions of an outflow with the medium. Some of these systems
are, or may be, high-mass binaries harboring young nonaccreting pulsars, in
which the stellar and the pulsar winds collide, generating a powerful outflow
that should terminate at some point in the ambient medium. This work studies
the evolution and termination, as well as the related radiation, of the
shocked-wind flow generated in high-mass binaries hosting powerful pulsars. A
characterization, based on previous numerical work, is given for the
stellar/pulsar wind interaction. Then, an analytical study of the further
evolution of the shocked flow and its dynamical impact on the surrounding
medium is carried out. Finally, the expected nonthermal emission from the flow
termination shock, likely the dominant emitting region, is calculated. The
shocked wind structure, initially strongly asymmetric, becomes a
quasi-spherical, supersonically expanding bubble, with its energy coming from
the pulsar and mass from the stellar wind. This bubble eventually interacts
with the environment on ~pc scales, producing a reverse and, sometimes, a
forward shock. Nonthermal leptonic radiation can be efficient in the reverse
shock. Radio emission is expected to be faint, whereas X-rays can easily reach
detectable fluxes. Under very low magnetic fields and large nonthermal
luminosities, gamma rays may also be significant. We conclude that the
complexity of the stellar/pulsar wind interaction is likely to be smoothed out
outside the binary system, where the wind-mixed flow accelerates and eventually
terminates in a strong reverse shock. This shock may be behind the extended
X-rays observed in some binary systems. For very powerful pulsars, part of the
unshocked pulsar wind may directly interact with the large-scale environment.Comment: 11 pages, 6 Figures, 1 Table, Astronomy and Astrophysics, in press
(small corrections after proofs
Tearing instability in relativistic magnetically dominated plasmas
Many astrophysical sources of high energy emission, such as black hole
magnetospheres, superstrongly magnetized neutron stars (magnetars), and
probably relativistic jets in Active Galactic Nuclei and Gamma Ray Bursts
involve relativistically magnetically dominated plasma. In such plasma the
energy density of magnetic field greatly exceeds the thermal and the rest mass
energy density of particles. Therefore the magnetic field is the main reservoir
of energy and its dissipation may power the bursting emission from these
sources, in close analogy to Solar flares. One of the principal dissipative
instabilities that may lead to release of magnetic energy is the tearing
instability. In this paper we study, both analytically and numerically, the
development of tearing instability in relativistically magnetically-dominated
plasma using the framework of resistive magnetodynamics. We confirm and
elucidate the previously obtained result on the growth rate of the tearing
mode: the shortest growth time is the same as in the case of classical
non-relativistic MHD, namely where is the
\Alfven crossing time and is the resistive time of a current layer.Comment: Submitted to MNRAS, few typos correcte
Star-Jet Interactions and Gamma-Ray Outbursts from 3C454.3
We propose a model to explain the ultra-bright GeV gamma-ray flares observed
from the blazar 3C454.3. The model is based on the concept of a relativistic
jet interacting with compact gas condensations produced when a star (red giant)
crosses the jet close to the central black hole. The study includes an
analytical treatment of the evolution of the envelop lost by the star within
the jet, and calculations of the related high-energy radiation. The model
readily explains the day-long, variable on timescales of hours, GeV gamma-ray
flare from 3C454.3, observed during November 2010 on top of a weeks-long
plateau. In the proposed scenario, the plateau state is caused by a strong wind
generated by the heating of the star atmosphere by nonthermal particles
accelerated at the jet-star interaction region. The flare itself could be
produced by a few clouds of matter lost by the red giant after the initial
impact of the jet. In the framework of the proposed scenario, the observations
constrain the key model parameters of the source, including the mass of the
central black hole: , the total jet power:
, and the Doppler factor of the
gamma-ray emitting clouds, . Whereas we do not specify the
particle acceleration mechanisms, the potential gamma-ray production processes
are discussed and compared in the context of the proposed model. We argue that
synchrotron radiation of protons has certain advantages compared to other
radiation channels of directly accelerated electrons.Comment: 16 pages, 5 figures, submitted to Ap
Orbital evolution of colliding star and pulsar winds in 2D and 3D; effects of: dimensionality, EoS, resolution, and grid size
(abridged)The structure formed by the shocked winds of a massive star and a
non-accreting pulsar in a binary suffers periodic and random variations of
orbital and non-linear dynamical origin. For the 1st time, we simulate in 3 D
the interaction of isotropic stellar and relativistic pulsar winds along 1 full
orbit, on scales well beyond the binary size. We also investigate the impact of
grid resolution and size, and of different EoOs: a gamma-constant ideal gas,
and an ideal gas with gamma dependent on temperature. We carry out, with the
code PLUTO, relativistic HD simulations in 2 and 3 D of the interaction of a
slow wind and a relativistic wind with Gamma=2 along 1 full orbit up to ~100 x
the binary size. The different 2-D simulations are carried out with equal and
larger grid resolution and size, and 1 of them is done with a more realistic
equation of state, than in 3 D. The simulations in 3 D confirm previous results
in 2 D. The shocked flows are subject to a faster instabilities growth in 3 D,
which enhances the presence of shocks, mixing, and large-scale disruption. In 2
D, higher resolution simulations confirm lower resolution results, simulations
with larger grid sizes strengthen the case for the loss of global coherence of
the shocked-wind structure, and simulations with 2 different EoOs yield very
similar results. In addition to the KHI, we find that the Richtmyer-Meshkov and
the RTI are likely acting together in the shocked flow evolution. Simulations
in 3 D confirm that the interaction of stellar and pulsar winds yields
structures that evolve non-linearly and get strongly entangled. The evolution
is accompanied by strong kinetic energy dissipation, rapid changes in flow
orientation and speed, and turbulent motion. The results strengthen the case
for the loss of global coherence of the shocked structure on large scales,
although higher pulsar wind speed simulations are needed.Comment: 13 pages, 12 figures, accepted for publication in Astronomy and
Astrophysic
On V_ud determination from kaon decays
The pion beta decay pi^+ -> pi^0 e^+ nu proceeds through pure weak vector
hadronic currents and, therefore, the theoretical prediction for it is more
reliable than for the processes with axial-vector current contribution. For
example, recently the pion beta decay has been used for V_ud determination. The
main aim of this letter is to point that kaon beta decay K^0 -> K^+(pi^+ pi^0)
e^- nu-bar analogously can be used for this purpose.Comment: 3 pages, no figures, one reference adde
Jets and gamma-ray emission from isolated accreting black holes
The large number of isolated black holes (IBHs) in the Galaxy, estimated to
be 10^8, implies a very high density of 10^-4 pc^-3 and an average distance
between IBHs of 10 pc. Our study shows that the magnetic flux, accumulated on
the horizon of an IBH because of accretion of interstellar matter, allows the
Blandford-Znajeck mechanism to be activated. Thus, electron-positron jets can
be launched. We have performed 2D numerical modelling which allowed the jet
power to be estimated. Their inferred properties make such jets a feasible
electron accelerator which, in molecular clouds, allows electron energy to be
boosted up to 1 PeV. For the conditions expected in molecular clouds the
radiative cooling time should be comparable to the escape time. Thus these
sources can contribute both to the population of unidentified point-like
sources and to the local cosmic ray (CR) electron spectrum. The impact of the
generated electron CRs depends on the diffusion rate inside molecular clouds
(MCs). If the diffusion regime in a MC is similar to Galactic diffusion, the
produced electrons should rapidly escape the cloud and contribute to the
Galactic CR population at very high energies >100 TeV. However, due to the
modest jet luminosity (at the level of 10^35 erg s^-1) and low filling factor
of MC, these sources cannot make a significant contribution to the spectrum of
cosmic ray electrons at lower energies. On the other hand, if the diffusion
within MCs operates at a rate close to the Bohm limit, the CR electrons
escaping from the source should be confined in the cloud, significantly
contributing to the local density of CRs. The IC emission of these
locally-generated CRs may explain the variety of gamma ray spectra detected
from nearby MCs.Comment: 6 pages, accepted by MNRA
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